Make DatastoreAccess::getItems const

I want to use this method in other const methods.

Change-Id: I32ff3fd8f0e77dbcea9b83b8ab8da99c1aa41f17
diff --git a/src/sysrepo_access.hpp b/src/sysrepo_access.hpp
index 4d87847..3150791 100644
--- a/src/sysrepo_access.hpp
+++ b/src/sysrepo_access.hpp
@@ -28,7 +28,7 @@
 public:
     ~SysrepoAccess() override;
     SysrepoAccess(const std::string& appname, const Datastore datastore);
-    Tree getItems(const std::string& path) override;
+    Tree getItems(const std::string& path) const override;
     void setLeaf(const std::string& path, leaf_data_ value) override;
     void createItem(const std::string& path) override;
     void deleteItem(const std::string& path) override;
@@ -43,7 +43,7 @@
 
 private:
     std::vector<ListInstance> listInstances(const std::string& path) override;
-    [[noreturn]] void reportErrors();
+    [[noreturn]] void reportErrors() const;
 
     std::string fetchSchema(const char* module, const char* revision, const char* submodule);
     std::vector<std::shared_ptr<sysrepo::Yang_Schema>> listSchemas();